Trends in Parasitology
ReviewLiving in a phagolysosome; metabolism of Leishmania amastigotes
Section snippets
The enemy within
Several important human microbial pathogens proliferate within macrophages (Mø). Although most of these pathogens evade the microbicidal responses of the Mø host by subverting or escaping from the phagocytic pathway, protozoan parasites belonging to the genus Leishmania manage both to survive and to proliferate within the mature phagolysosome compartment of Mø. The infection of Mø by Leishmania is initiated by flagellated, metacyclic (non-dividing) promastigotes, which are injected into the
Nutrient availability and uptake in the phagolysosome
Intracellular stages of Leishmania must scavenge all of their carbon source and micronutrient requirements from the lumen or limiting membrane of the Mø phagosome (Figure 1). Based on the known nutrient requirements of cultured stages [5] and genome-based reconstructions of the Leishmania metabolome 6, 7, it is predicted that Leishmania amastigotes must scavenge all their purine requirements, many vitamins and at least ten essential amino acids from the Mø phagolysosome (Figure 2). The uptake
Use of stable mutants to identify metabolic pathways required for virulence
The genetic disruption of specific metabolic pathways is the most rigorous approach for demonstrating a requirement for intracellular growth (but see Box 1 for pros and cons). The Leishmania mutants listed in Table 2 can be classified into three categories. PAV (promastigote avirulent) mutants are poorly infective as promastigotes, but fully infective if viable amastigotes can be generated. These mutants characteristically generate lesions after a delay of several weeks. AAV (amastigote
Gene and protein expression profiling of amastigote stages
Transcript and protein profiling approaches have been used to probe the physiological state of many intracellular pathogens, often revealing the concerted up- and downregulation of entire metabolic pathways, such as glyoxylate shunt and gluconeogenesis [25]. However, Leishmania and other trypanosomatids lack a conventional network of transcription factors and most genes are constitutively transcribed [43]. Although there are mechanisms for regulating mRNA levels in a stage- or growth-dependent
New approaches for defining the physiological state of intracellular pathogens
Changes in the physiological state of amastigotes can also be assessed by measuring the steady-state levels of cellular metabolites. Metabolite profiling, or metabolomics, refers to the quantitative analysis of low molecular weight metabolites (Box 2), and is increasingly being used to complement transcript and protein-profiling approaches, as well as being an important technique in its own right [52]. Metabolite profiling approaches might be particularly useful for assessing the physiological
Conclusions and perspectives
It is often assumed that the metabolic repertoire of microbial pathogens is the result of adaptations to nutrient conditions encountered in their respective hosts. An alternative (and not mutually exclusive) view is that the metabolic repertoire of microbial pathogens might define the range of niches in the host that can be successfully colonized. We suggest that the complex nutritional requirements of Leishmania and their inability to use fatty acids as their primary carbon source has severely
Acknowledgements
We were unable to reference all relevant work owing to space constraints. Our laboratory is supported by the Australian National Health and Medical Research Council.
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